1. Field of the Invention
The present invention relates to an suppressing field radiation from a display device.
2. Description of the Background Art
In a visual display terminal (VDT) employing a cathode ray tube, field radiation, particularly that in a range of 5 Hz to 400 kHz, problematically exerts a harmful influence upon the human body. According to the MPR standards (MPR 1990) in Sweden, for example, alternating electric fields (AEF) are regulated not to exceed 25 V/m and 2.5 V/m in BANDs I (20 Hz to 2 kHz) and II (2 kHz to 400 kHz) respectively
It has been recognized that an electric field which is radiated from a VDT is generally caused by flyback pulses of the horizontal deflection circulating, and this electric field is mainly generated from a deflection yoke which is mounted on a cathode ray tube. Such an electric field which is caused by flyback pulses for horizontal deflection appears in a relatively high frequency domain in the aforementioned frequency range. In addition to the aforementioned electric field based on flyback pulses for horizontal deflection, the VDT also radiates an electric field caused by a high voltage, which is applied to an anode of the cathode ray tube, fluctuating in vertical synchronization. Such an electric field which is caused by the voltage fluctuating in vertical synchronization appears in a relatively low frequency domain in the aforementioned frequency range.
In a region other than the front surface of the cathode ray tube, the human body can be effectively shielded against radiation of such an electric field by a shielding member made of a metal plate or the like.
In a region in front of the cathode ray tube, however, the human body cannot be simply shielded by such a shielding member as a metal plate or the like, since it is necessary to visually display images on the front surface.
In order to prevent the front surface of the cathode ray tube from being charged with a high applied voltage thereby protecting the front surface from dust in the air, on the other hand, a transparent conductive film is formed on the front surface of the cathode ray tube, or a transparent panel is arranged in front of the cathode ray tube, and grounded. Such a transparent conductive film can shield the human body against an electric field which is radiated from the front surface of the cathode ray tube to some extent. However, a high cost is required for forming such a transparent conductive film, whose shielding effect is rather insufficient considering its cost.
While a liquid crystal display device is regarded as having a smaller field radiation as compared with a display device employing a cathode ray tube, electric fields are problematically radiated from a back light, an invertor, an alternatingly driven display panel and the like which are included in the liquid crystal display device.
In addition to the liquid crystal display device, such a problem is also caused in an alternatingly driven flat display device such as a plasma display device, for example.